The motor neuron regulates many of the physiological and biochemical properties of the muscle cells it innervates. The major objective of this proposal is to define how this long-term nerve-muscle interaction operates. The proposal has two facets: One is the question of whether long-term regulation of muscle properties is simply the result of synaptic activation and its sequelae (muscle cell depolarization, action potential, contraction) or whether it reflects a special neurotrophic influence. These alternatives will be tested in an organ culture system using paired rat or mouse muscles, one with and the other without a long nerve stump. There is preliminary evidence that under these conditions the presence of a long nerve stump alters muscle amino acid transport and incorporation into protein. If these parameters are regulated by nerve or muscle electrical activity, it will be revealed by experiments in which electrical activity and spontaneous transmitter release are monitored or altered. If axonal transport of trophic substances is implicated, the in vitro system will be useful as a bioassay for the identification of these substances. The second related question is whether the muscle first responds to neuronal influence or to denervation by changes in surface proteins. In order to answer this question in molecular terms, a method for radioactive labelling and characterization of muscle surface proteins has been developed. With this method, we will be able to determine if denervation in vivo leads to the appearance of new or modified surface protein species or increased surface protein turnover. The organ culture system will be useful here both for long-term incorporation studies and for investigation of neuronal mechanisms as proposed above.